240 P. L. Gersper et al. 



in snowfall declined from almost 30% to less than 10% between early 

 September and late October of 1960 (Dugdale and Toetz 1961). Nitrite 

 concentrations in fresh snow are exfemely low, with an upper limit of 

 about 1 /ig N liter"'. Comparisons of these values with nitrogen distribu- 

 tion in snow columns in May indicate that ammonium may be converted 

 to nitrate in the snowpack. The concentration of inorganic nitrogen in 

 both samples was similar, approximately 80 ^g liter"', but the concentra- 

 tion of nitrate in the spring sample was higher by 15 /:4g N liter"'. Concen- 

 trations of all three inorganic forms of nitrogen are higher in rain than in 

 snow. The total concentration of nitrogen in summer precipitation is 340 

 /ig hter"', of which ammonium contributes 15% and organic nitrogen less 

 than 20<Vo (Prentki et al. 1980). 



The yearly input of nitrogen by precipitation was calculated by Bars- 

 date and Alexander (1975) as 23.4 mg m"^ However, revised values of 

 total snowfall (Chapter 2) and the inclusion of organic nitrogen indicate 

 that this value should be raised to 30.5 mg m'^ yr"'. The majority of this 

 input occurs during the summer. Although the amount of nitrogen in 

 precipitation is very small in comparison with the total nitrogen pool, 

 most of this nitrogen enters the system in inorganic forms and supplies 

 an amount equal to \% of the inorganic pool in the upper 10 cm of the 

 soil. The nitrate content of precipitation seems particularly large from 

 this viewpoint, more than seven times greater than the nitrate pool in the 

 soil. 



Precipitation is the major external source of phosphorus for the soil 

 of the coastal tundra at Barrow (Table 7-5). As with nitrogen, phosphor- 

 us concentrations are lower in snow than in summer precipitation, 4.0 vs 

 7.9 ^g hter"' (Prentki et al. 1980); slightly more than half of the total in- 

 put occurs during the summer. The inorganic phosphorus added by pre- 

 cipitation is equal to 6% of the labile phosphorus pool and is actually 

 larger than the amount of dissolved inorganic phosphorus. The input of 

 phosphorus in precipitation thus may be important in supplementing the 

 small amounts of available phosphorus in the soil as well as in counter- 

 acting long-term losses to runoff. 



Loss of Nitrogen and Phosphorus in Runoff 



The major pathway for nitrogen outflow from the coastal tundra at 

 Barrow is in surface runoff during the brief period of snowmeh (Table 

 7-5). The portion of winter precipitation that runs off during this time 

 varies from as low as 51*'7o at the Biome pond site to 95% at Esatkuat 

 Creek (Miller et al. 1980). To produce a generalized nitrogen and phos- 

 phorus budget for the Barrow area, an intermediate runoff value of 83% 

 was selected which corresponds to the spring runoff at the Biome pond 



